Docs: the field first, and the morph as something you turn on (to main)

[ronickg]

#6 was merged into docs/worklet-events twenty minutes after #3 had already taken that branch to main, so its content never reached main. This is the same work, merged onto main cleanly (no conflicts with #4 or #5), plus the one row that was waiting on both: signPlacement on the props page.

What #6 does, for reference:

• Getting started opens on the plain field (live demo, no morph), then an amount and a frame that do not animate, and says up front that the morph is an effect you turn on.
• An amount field is about the formatting. The demo has a Plain / Morph switch, off by default. Gains the keyboard rules, limits, currency layouts and shrink-to-fit that were README-only.
• The morph (new, under "Opt-in effects") takes over the text demo and the tuning props. /input/text and /input/plain-field redirect.
• Frames (new): the floating label, the notch as a real hole in the stroke, stroke / fill / corners, a live outlined ↔ filled demo, and why it is native rather than an animated label over a TextInput. No Material references.
• Multiline (new): numberOfLines / rows, growing, scrollEnabled, textAlignVertical, lineHeight.
• Props grouped with the morph rows marked opt-in; Working with React Native gains threading and motion; landing copy no longer calls the input "a field that morphs".
• README: a multiline section and rows, "single-line" dropped, Material replaced with the reason the label is native. worklets.ts module comment lists all eight handlers.

npm run build in docs/ passes on the merged result; every link and anchor resolves.

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